<span>Gas injection into supersonic flow past a 5-degree half-angle cone is studied with three injected gases: helium, nitrogen, and RC318. Experiments are performed in a Mach 4 Ludwieg tube with nitrogen as the free stream gas. The injector section is shaped to admit a "tuned" injection rate where the displacement created by injection counteracts the effects created by the injector geometry. A high-speed schlieren imaging system with a framing rate of 290 kHz is </span><span>used to study the instability in the region of flow downstream of injection, referred to as the injection layer. Measurements of wavelength, convective speed, and frequency of the instability waves were made. The stability characteristics of the injection layer are found to be very similar to those of a shear layer. The findings of this work suggest that shear layer modes should be a primary concern for future stability analyses of supersonic flow with injection.</span>
